In a transmission electron microscope (TEM) a beam of high energy (typically 20-300 keV) electrons is directed onto a thin specimen. The electrons that are scattered by the specimen are analyzed, along with other secondary signals that are produced by the interaction of the incident electrons with the specimen. The electrons which pass through the specimen (the transmitted electrons) are scattered in an angular distribution, which is dependent on the atomic structure of the specimen, the thickness of the specimen, the energy of the electrons and the relative orientation between the atomic structure and the electron beam direction. The measured angular distribution of the transmitted electrons is referred to as the electron diffraction (ED) pattern, because the shape of the pattern is the result of interference of the electrons scattered from different atoms in the specimen in a process known as diffraction. ED patterns can be used to determine the atomic structure of materials.
The key advantage of a TEM in producing ED patterns is the possibility of focusing the incident electrons into a small area with dimensions approaching one nanometer, thus enabling the analysis of atomic structure from nanometer-sized regions. Furthermore, with the ability to systematically move the small focused incident electron beam to different positions on the specimen in a scanning transmission electron microscope (STEM), it is possible to analyze the spatial variations in atomic structure due to material characteristics such as polycrystalline grain structure elastic and plastic strain. These measurements can be made with a spatial resolution of one nanometer or better.
Automated analysis of ED patterns from crystalline materials is complicated by an effect known as dynamical diffraction, which arises from a complex interaction between many electrons scattered in different directions. This effect is strongly dependent on specimen thickness and by the relative orientation between the electron beam direction and the specimen's crystal axes. The net effect is that small variations in crystal thickness and orientation lead to very large changes in the ED pattern. This complicates analysis of the ED patterns because there is no independent way to measure the thickness or orientation of the specimen.
Precession electron diffraction (PED) was invented to simplify automated analysis of ED patterns. PED is based on the fact that the dynamical diffraction effects are independent of the incident beam direction, whereas other diffracted intensity (the so-called kinematical diffraction) is relative to the incident beam direction. Therefore, if the incident electron beam is tilted, the kinemetical part of the ED pattern shifts (since the ED pattern is an angular distribution), whereas the dynamical part of the pattern doesn't move. Then if the diffraction pattern is shifted in order to keep the kinematical diffraction pattern fixed while the beam is tilted (a process known as “tilt descanning”), the dynamical diffraction intensity shifts. If the tilt angle of the electron beam is changed rapidly in some pattern while recording an ED pattern, and the ED pattern is descanned to keep the kinematical part of the pattern fixed, the dynamical part of the pattern is “smeared out” and becomes a relatively uniform background behind the kinematical part of the pattern (which is generally a set of small bright spots).
The most common pattern in which the electron beam is tilted is a so-called precession pattern, where the tilt angle relative to the microscope optical axis (the precession angle) is fixed, and the tilted beam is rotated about the optical axis by changing the azimuthal angle at a constant rate. The beam azimuth is typically rotated through an entire circle (360 degrees) during the recording of one ED pattern, which is then referred to as a PED pattern.
The beam is shifted and tilted In a TEM/STEM by a two-stage deflection system, where two deflectors are positioned centered around the optical axis at different distances from the specimen upstream of where the incident beam enters the pre-field objective lens. The deflectors can be either electromagnetic or electrostatic. The balance between the two deflector stages is a fixed value (one for shift and another for tilt) which defines the beam shift/tilt “purity”. Additionally, there are two pairs of deflectors at 90 degree angles with respect to each other which can produce shift and tilt in two orthogonal directions. Finally, there are two pairs of two-stage deflectors on the exit side of the specimen for descanning the beam shift and tilt.
In most existing TEM/STEMs, a single set of two-stage deflection coils are used for both beam shift and for beam tilt, like in U.S. Pat. No. 8,253,099B2. The signals which drive each of the shift and tilt deflectors are added before being sent to the deflectors. A limitation of this arrangement is that the strength of the deflectors needed for shift and for tilt can be very different (by 1-2 orders of magnitude). This makes it difficult to add the shift and tilt signals and still maintain the necessary dynamic range in the summed signal to accurately represent both signals. Additionally, the characteristic frequencies of the shift and tilt signals are often very different, so it is difficult to design a single deflector circuit with the proper frequency response to accurately produce both the shift and tilt deflections. Finally, the optimum positions for the beam shift deflectors might not be at the same position as the deflectors for beam tilt.
Although both scanning and precession movements may be provided using a STEM (simultaneously or separately), the needs concerning deflection amplitude, stability and dynamic range are different.